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The increased use of diesel engines for light-duty and heavy-duty vehicle applications and the associated uncertainties regarding exhaust emissions from these engines on human health has led to focused attention on risk assessments of diesel engine exhaust. Diesel particulate matter, in particular, has been associated with adverse health effects. Some recent epidemiological studies suggest a weak correlation between elevated particulate matter concentration in ambient air and cardiopulmonary health effects. The correlation is even stronger for smaller combustion generated particles (PM2.5). Mortality is mostly due to respiratory and cardiac problems. As associations between particulate matter and respiratory disorders are evidenced more clearly, the need to fully understand and quantify vehicle particle emissions becomes crucial. Compressed and liquefied natural gas fueled engines have significantly reduced total mass emission rates, but at the same time, have contributed a larger portion of this total is ultra-fine particles. In the past two year West Virginia University (WVU) has characterized particle size distribution of particulate matter from over 60 in-use heavy-duty vehicles. Conventional diesel, and several alternative fuels fueled these vehicles, and many of these vehicles were equipped with exhaust after-treatment devices. WVU's Transportable Heavy-Duty Vehicle Emissions Testing Laboratory (THVETL) was used at various sites to test the recruited in-use vehicles. Herein, PM sizing distribution methods and protocols will be discussed and size-distribution data from few of the vehicles test will be presented.